A problem prevalent in the generation of electronic data, especially, image or video data, is maintaining a calibrated scanner. The calibration process is used to correct for offset errors and gain errors in the video signal. Offset and gain errors can be caused by the characteristics of individual components responsible for converting the light reflected from the image into electronic image data. These errors may result from nonuniformities in the characteristics of the illumination and sensing components in the scanner, as well as from time varying drifts of such characteristics. For example, a charged coupled device (CCD) sensor may have inherent offset and gain characteristics unique to itself or a scanner may contribute to offset and gain errors due to the present operating conditions such as the operating temperature, lamp color temperature, age, etc. If offset errors or gain errors are not adequately addressed; i.e., the signal being processed is not adjusted to counteract the offset or gain errors; the processing of the signal will not be accurate which, in an image processing system, can cause the generated picture or image to have a lower quality.
To address these problems, typical image processing systems or image scanning systems perform calibrations of the imaging system at intervals. Often, the calibration is performed at power-up but, in some cases, calibration is performed at fixed intervals, sometimes as frequently as each scan. Following are examples of systems which perform calibration routines.
An example of a device which performs calibration once every predetermined number of scans is the device disclosed in U.S. Pat. No. 3,952,144 to Kolker. Kolker discloses that a facsimile transmitter makes a preliminary calibrating scan in which the transmitter sequentially scans a known black area and a known white area. An automatic background and contrast control unit stores a first sample of the uncorrected video signal which represents the scanned black area and stores a second sample of the uncorrected video signal which represents the scanned white area. During subsequent scanning, the automatic background and contrast control unit continually produces voltages representing the stored black and white samples and uses these voltages to correct the video signal received during the scanning of the document.
Another example of a device which corrects for offset and gain errors is disclosed in U.S. Pat. No. 4,555,732 to Tuhro. This U.S. Patent discloses an image sensor correction system which maintains the offset voltages in the shift registers of a multi-channel image sensor substantially equal. U.S. Pat. No. 4,555,732 discloses that a pair of control gates permits sampling the existing offset voltages in the shift register of each channel to provide an adjusted potential for balancing any voltage differences between the shift registers. More specifically, U.S. Pat. No. 4,555,732 discloses a device which compares the various offsets of a plurality of shift registers and determines a single offset potential to be applied to each shift register according to the comparison.
A device which proposes to correct gain and offset errors due to changes in the operating characteristics of a CCD is disclosed in U.S. Pat. No. 4,216,503 to Wiggins. U.S. Pat. No. 4,216,503 discloses a system where dark and light level signals are isolated and processed by a microprocessor unit in accordance with a pre-established routine to provide an offset potential and gain multiplicand. The determined offset potential and gain multiplicand are used to remove the offset and set a signal gain for the next succeeding line of image signals. The process is then repeated for each line of image signals to be outputted from the CCD.
Although U.S. Pat. No. 4,216,503 discloses a device to correct offset and gain errors on a continual basis, such a process is not adaptable to correct offset or gain errors in a high speed copier or errors in a fast scan direction parallel to the CCD because this method only corrects for offset errors or gain errors in a slow scan direction perpendicular to the CCD. In other words, the technique disclosed by U.S. Pat. No. 4,216,503 adjusts the offset gain value only upon the completion of a scanning of a full line of data.
The calibration of a conventional digital scanner is illustrated in FIG. 4. An optical system is positioned so as to scan a strip of calibration material 3. More specifically, during a calibration process, the digital scanner causes a lamp 1 to illuminate both a dark area 13 and a white area 12 of a calibration strip 3. The light reflected from the calibration strip 13 is directed towards a sensor array 7 which may be either a CCD sensor or a full width array sensor. The sensor array 7 converts the light into electrical signals corresponding to digital image data representing the image that has been scanned, namely the image of the calibration strip 3. This image data is then fed to a calibration circuit 9 which with information received from a nonvolatile memory 20 produces offset and gain correction data for each sensor pixel. The information received from the nonvolatile memory 20 includes the predetermined or pre-measured reflectance values of the several portions of the calibration strip 3. The calibration circuit 9 then produces offset and gain correction data by comparing the signal from each of the sensor's pixels with the predetermined value for the calibration strip. This correction data is stored in memory and used to correct the gain and offset of each pixel during normal scanning operation, thus providing a signal representative of image data which is fully calibrated and corrected for any errors in the sensor array or variations in the lamp's properties.
A problem associated with calibration is the establishment of the reference reflectance values of the calibration strip. During calibration of digital scanners as noted above, an optical system (a lamp 1, sensor array 7, and lens 14) is positioned, as illustrated in FIG. 4, to allow the sensor array 7 to view a strip of calibration information and/or patterns 3 located on or near the platen. It is important that an accurate value of the reflectance of this calibration strip 3 be known to the digital scanner to achieve an absolute reflectance calibration and generate accurate correction values for subsequent document reading operations. Unfortunately, the variation in calibration strip reflectance, particularly, batch to batch, is presently greater than is required to achieve the desired calibration accuracy.
To counter this variation in the calibration strip reflectance, some conventional digital scanners include a writable portion of the nonvolatile memory 20 which stores the measured strip's reflectance. Thus, it is necessary to manually enter the measured strip's reflectance into the conventional scanner's nonvolatile memory 20 each time a new calibration strip 3 is installed, either during initial assembly in the factory or during replacement in the field. The requirement of manually entering the measured strip's reflectance into the scanner's nonvolatile memory 20 adds cost to the assembly and field service of the digital scanner and it presents opportunities for error which can significantly impact the digital scanner's image quality.
The present invention provides a machine readable tag or portion with each calibration strip that has encoded thereon the strip's reflectance values so that the scanner can automatically read the strip's reflectance values during a calibration routine. This eliminates the need of a user or technician to manually enter the information.